A fuel cell, such as that presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 101 between them. Here the structure is called an electrolyte element 104 (FIGS. 2, 3, 4). In solid oxide fuel cells (SOFCs) oxygen 106 is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion transfers through the electrolyte material 101 to the anode side 100 where it reacts with fuel 108 thereby producing electrons, water and also for example, carbon monoxide (CO) and carbon dioxide (CO2). Anode 100 and cathode 102 are connected through an external electric circuit 111 having a load 110 for the fuel cell withdrawing electrical energy alongside heat out of the system. The fuel cell reactions in the case of methane, carbon monoxide and hydrogen fuel are shown below:Anode: CH4+H2O=CO+3H2 CO+H2O=CO2+H2 H2+O2−=H2O+2e−Cathode: O2+4e−=2O2−Net reactions: CH4+2O2=CO2+2H2OCO+1/2O2=CO2 H2+1/2O2=H2O
In an electrolysis operating mode (solid oxide electrolysis cells (SOEC) the reaction is reversed; i.e., heat, as well as electrical energy from a source 110, are supplied to the cell where water and often also carbon dioxide are reduced in the cathode side forming oxygen ions, which move through the electrolyte material to the anode side where oxidation reaction takes place. It is possible to use the same solid electrolyte cell in both SOC and SOEC modes. In such a case and in the context of this description the electrodes are for example, named anode and cathode based on the fuel cell operating mode, whereas in purely SOEC applications the oxygen electrode may be named the anode, and the reactant electrode as the cathode.
Solid oxide electrolyzer cells operate at temperatures which allow high temperature electrolysis reaction to take place, the temperatures being for example, between 500-1000° C., but temperatures differing from these limits may be useful. These operating temperatures are similar to those conditions of the SOFCs. The net cell reaction produces hydrogen and oxygen gases. The reactions for one mole of water are shown below, with reduction of water occurring at the anode:Cathode: H2O+2e−- - - >2 H2+O2−Anode: O2−- - - >1/2O2+2e−Net Reaction: H2O - - - >H2+1/2O2.
In Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolyzer (SOE) stacks, commonly here referred to as solid oxide cell stacks, where the flow direction of the cathode gas relative to the anode gas internally in each cell as well as the flow directions of the gases between adjacent cells, are combined through different cell layers of the stacks. Further, the cathode gas or the anode gas or both can pass through more than one cell before it is exhausted and a plurality of gas streams can be split or merged after passing a primary cell and before passing a secondary cell. These combinations serve to increase the current density and minimize the thermal gradients across the cells and the whole stack.
A SOFC delivers in normal operation a voltage of approximately 0.8V. To increase the total voltage output, the fuel cells can be assembled in stacks in which the fuel cells are electrically connected via flow field plates (also: separator plates, interconnect plates, interconnector plates, bipolar plates). The desired level of voltage determines the number of cells needed.
Bipolar plates separate the anode and cathode sides of adjacent cell units and at the same time enable electron conduction between anode and cathode. Interconnects, or bipolar plates can be provided with a plurality of channels for the passage of fuel gas on one side of an interconnect plate and oxygen rich gas on the other side. The flow direction of the fuel gas is defined as the substantial direction from the fuel inlet portion to the fuel outlet portion of a cell unit. Likewise, the flow direction of the oxygen rich gas is defined as the substantial direction from its inlet portion to its outlet portion of a cell unit.
Known cells are stacked one on top of each other with a complete overlap resulting in a stack with for instance co-flow having all fuel and oxidant inlets on one side of the stack and all fuel and oxidant outlets on the opposite side. One feature affecting the temperatures of the structure in operation is steam reformation of the fuel that is fed into the cell. Steam reformation is endothermic reaction and cools the fuel inlet edge of the cell.
Due to the exothermicity of the electrochemical process, the outlet gases leave at a higher temperature than the inlet temperature. When endothermic and exothermic reactions are combined in an SOFC stack a significant temperature gradient across the stack is generated. Large thermal gradients induce thermal stresses in the stack which are highly undesirable and they entail a difference in current density and electrical resistance. Therefore challenges of thermal management of an SOFC stack exist: to reduce thermal gradients enough to avoid unacceptable stresses and to maximize electric efficiency through homogenous current density profile.
Known fuel cell stacks or electrolyzes cell stacks have tolerance variations in electrolyte element structure thickness between the cell structures in the stacks. For example in a cell stack structure, in which ceramic materials are used, only thickness variations in the measure of only micrometers would be convenient in known embodiments. This results in differential flowing conditions between the cells causing varying cell voltage profile in the stack structure resulting in thermal gradients between the cells and decreased power density of the stack. Thus both the duty ratio of the stacks is decreased, and lifetime of the stacks is shortened, the first increasing the capital cost of the stack per produced electrical power output and the later increasing the operational cost of the stack structure as for example, the stack replacement time is shortened in a fuel cell system and cost of electricity is increased in the electrolyzer stack.
High temperature solid oxide cell stacks are alternate conversion technologies due to their extream high efficiencies both in fuel cell and electrolysis mode. The inherent challenge related to these technologies also stems from the high temperature, the challenge being corrosion of the materials causing increasing internal resistances to the structures decreasing the electricity production and hydrogen production capability of the fuel cell and the electrolyzer, respectively. Corrosion can exist in multiple places of the stack structure but are for example, emphasized in regions containing various material systems. Such a system is the triple phase area between metallic interconnect structure, sealing structure and oxidizing gas. In such a material system, for example, the metallic interconnect material, which is for example, made of ferritic stainless steel grades due to its good corrosion resistance and matching thermal expansion characteristics between other stack materials, can react with the sealing structure made for example, from at least partly glass material by for example, changing the crystal structure of the metal or by changing the protective oxide structure of the metal surface which eventually may lead to through plane oxidation of the steel material creating a direct path for fuel and oxygen to mix causing a catastrophic failure of the structure.